Legionella pneumophila fails to succomb to the antimicrobial action of polymorphonuclear leukocytes (PMN). The effective countermeasure employed by the organism to combat PMN antimicrobial action has not been elucidated. One possibility is that an extracellular product produced by the bacteria assists in the process. L. pneumophila synthesizes a low molecular weight peptide toxin which inhibits the activation of a PMN membrane associated enzyme, NADPH oxidase. The formation of oxygen derived antimicrobial agents by the PMN is thereby restricted and as a consequence, bacterial killing is significantly impaired in toxin-treated PMN. In order to more thoroughly evaluate the role of the toxin in securing the intracellular survival of L. pneumophila, we propose to first purify the toxin using reverse phase HPLC. We will also initiate studies aimed at delineating the toxin-induced block in the NADPH oxidase activation process. Evidence suggests that the polyphosphoinositide pathway toxin to inhibit activation of this pathway will be evaluated using a number of chemical stimulators. Finally, we intend to try and correct the bactericidal lesion in toxin-treated PMN, agents exist which can induce these cells to generate oxygen derived antimicrobial agents. We propose to expose toxin-treated PMN to chemical stimulators which are capable of eliciting oxidase activation. The bactericidal ability of these cells will then be evaluated.